1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-function system including a combination thereof, and more particularly, to a fixing device using an electromagnetic induction heating method and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical scanner projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Fixing devices that use an electromagnetic induction heating method to reduce a warm-up time (the time it takes the fixing device to reach a target temperature) of the image forming apparatus, thereby conserving energy, are known, such as JP-2009-14972-A. One example of such a fixing device using the induction heating method is equipped with a support roller (a heating roller) serving as a heat generating body, a fixing auxiliary roller (fixing roller), a fixing belt, an induction heater, and a pressing roller. The fixing belt is formed into a loop and wound around the support roller and the fixing auxiliary roller. The pressing roller contacts the fixing auxiliary roller via the fixing belt. The induction heater is disposed opposite the support roller via the fixing belt, and consists of a coil portion including an excitation coil, a core (excitation coil core) facing the coil portion, and a holder that holds parts such as the coil portion and the core. The excitation coil is wound longitudinally around the induction heater.
As the fixing belt rotates and comes to face the induction heater, the fixing belt is heated by the induction heater. Subsequently, the heated fixing belt heats a toner image on a recording medium at a fixing nip where the fixing auxiliary roller and the pressing roller meet and press against each other and through which the recording medium sheet is conveyed, thereby fixing the toner image onto the recording medium. More specifically, an alternating magnetic field is formed around the coil portion by supplying a high-frequency alternating current thereto. As a result, an eddy current is generated near the surface of the support roller, generating Joule heat through the electrical resistance of the support roller itself, which in turn heats the fixing belt wound around the support roller, accordingly.
In this configuration, the heat generating body is directly heated by electromagnetic induction, hence providing high heat conversion efficiency compared with other known heating methods such as those employing a halogen heater. The electromagnetic induction heating method can heat the surface of the fixing belt to a desired temperature (fixing temperature) quickly with little power.
Another example of a known fixing device using the electromagnetic induction heating method (JP-3519401-B) includes a core (i.e. back surface core) disposed opposite an excitation coil consisting of a C-type core and a center core to enhance heat generating efficiency.
Generally, in the fixing device using the electromagnetic induction heating method, a magnetic circuit needs to be closed to prevent generation of leakage flux from the coil for efficient induction heating. A known technique to close the magnetic circuit includes adding a ferrite core, a shield, or the like. The fixing device using the C-type core and the center core disposed opposite the excitation coil may enhance the heat generating efficiency of the heat generating member. However, the heat generating efficiency may not be sufficient.
In the known fixing devices described above, the heating member and the magnetic core that directs the magnetic flux from the excitation coil to the heat generating member are relatively widely separated, resulting in a longer time to bring the heat generating member to a desired temperature. In other words, the warm-up time of the fixing device is lengthened.
In view of the above, there is demand for an induction heating-type fixing device with good heating efficiency and a short warm-up time.